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A108896
Numbers whose outer two digits are 9's and inner digits are 4's.
1
949, 9449, 94449, 944449, 9444449, 94444449, 944444449, 9444444449, 94444444449, 944444444449, 9444444444449, 94444444444449, 944444444444449, 9444444444444449, 94444444444444449, 944444444444444449, 9444444444444444449, 94444444444444444449
OFFSET
1,1
COMMENTS
All terms are composite.
From Sergio Pimentel, Jul 26 2022: (Start)
a(n) is divisible by:
13 if n == 1 (mod 6).
11 if n == 0,2,4 (mod 6).
3 if n == 0,3 (mod 6).
7 if n == 5 (mod 6). (End)
FORMULA
a(n) = 9*10^(n+1) + 9 + 40*(10^n-1)/9.
From Chai Wah Wu, Jul 27 2022: (Start)
a(n) = 11*a(n-1) - 10*a(n-2) for n > 2.
G.f.: x*(949 - 990*x)/((x - 1)*(10*x - 1)). (End)
MATHEMATICA
Table[10*FromDigits[PadRight[{9}, n, 4]]+9, {n, 2, 20}] (* Harvey P. Dale, Apr 02 2018 *)
PROG
(PARI) S(n, r, m)=for(x=1, n, y=m*10^(x+1)+m+r*10*(10^x-1)/9; print1(y", "))
(Python)
def a(n): return int('9' + '4'*n + '9')
print([a(n) for n in range(1, 19)]) # Michael S. Branicky, Jul 26 2022
CROSSREFS
Sequence in context: A020366 A217936 A013532 * A260174 A285157 A096954
KEYWORD
easy,nonn,base
AUTHOR
Cino Hilliard, Jul 16 2005
STATUS
approved